The present invention relates generally to a zoom lens and an imaging system comprising it, and more particularly to a fast zoom lens that works for slimming down upon received in an associated lens mount. The present invention is also concerned with an imaging system such as digital camera or video camera, which comprises an electronic imaging device adapted to convert an image formed on an imaging plane into electrical signals.
As recently developed digital cameras diminish in size, there is a growing demand for size reductions of the cameras when not in use; in particular, there is a strong demand for size reductions in the thickness direction. As a result, imaging optical systems for such cameras, too, are required to be received in their lens mounts thinly in the thickness direction when not in use.
On the other hand, as cameras become much smaller and imaging devices have much more pixels, the pixel pitch of the imaging devices grow by far finer, offering problems such as severe limits to sensitivity due to SN ratio drops, the likelihood of camera shakes and subject shakes, and so on.
There is also now on the market a great demand for cameras that can just only address such shake problems but also cope with a lot more photographic applications than would be possible with prior art cameras and can take images of low-luminance subjects as well.
Possible approaches to these problems include a method of reducing camera shakes by means of a mechanical correction mechanism, a method of enhancing the sensitivity of an imaging device, thereby achieving faster shutter speeds, or the like.
With the method of correcting shakes by mechanical movement of a part of taking lenses or an imaging device, however, there is an extended exposure time that results in the inability to cope with subject shakes. To address this, there is a complicated control mechanism needed (for instance, the need of moving a taking position following a subject movement, the need for control of images by electrical computation processing after taking).
On the other hand, imparting high sensitivity to the imaging device may cope with both subject shakes and camera shakes; however, there is image quality degradation likely to be caused by noises from the imaging device.
Therefore, if the F-number of a taking lens is made so small (fast) that the quantity of light incident on the imaging device is increased, it would be effective for providing a solution to these problems.
With a prior art fast zoom lens having a minimum F-number of as small as 1.8, however, there is still much difficulty in ensuring that it is received in the associated lens mount thinly enough to be mounted on a compact camera.
Therefore, the primary object of the invention as described below is to provide a fast zoom lens having an F-number small enough to be mounted on a slimmed-down compact camera.
As well known generally in the art, the zoom lens type that the first lens group has positive refracting power is favorable for those having a small F-number. However, this type of zoom lens is found to work against being stowed away in the associated lens mount when not in use, because the first lens group grows large in the diametrical direction, and tends to increase in the number of lenses involved.
Optical system arrangements known to be put away compactly when not in use include the zoom lens type wherein the first lens group has negative refracting power.
This type of zoom lens, because of being put away more thinly than a zoom lens of the type having a lens group of negative refracting power first, is used with most slimmed-down compact cameras.
With conventional zoom lenses of the type having the lens group of negative refracting power first, however, there is still no sensible tradeoff between size reductions upon stowed in the lens mounts and a small F-number.
For instance, the zoom lenses of Patent Publication 1 that would seem to comprise fewer lenses and make sure a slimmed-down lens mount have an F-number of as large as about 2.9. For this reason, they are not suited for obtaining plenty of light quantity.
On the other hand, all zoom lenses having a small F-number, described in Patent Publications 2, 3, 4 and 5, comprise a lot more lenses, and so have difficulty in being stowed away in the associated lens mounts.
Patent Publication 1
JP(A)2004-318099
Patent Publication 2
JP(A)4-114116
Patent Publication 3
JP (A)1-40913
Patent Publication 4
JP(A)2001-42218
Patent Publication 5
JP(A)2001-208969